Flexographic, lithographic, gravure, screen printing, digital printing such as inkjet or drop on demand, and other processes, including those in which a central impression cylinder is used, are known in the art for printing a wide variety of articles. Recently, these and other processes and techniques have been used to print functional materials, including those required to make electrical circuits. Whereas etching and other subtractive processes for creating circuits are wasteful, relatively costly, and time-consuming, thereby limiting applicability and use, printing processes, by virtue of being additive, can be comparatively economical, efficient in the use of resources, and highly effective in meeting sufficient product quality standards, i.e., resolution, registration, and other indicators, for many applications. Resultant printed or otherwise selectively deposited products are therefore more attractive for a range of applications and industries, and especially where performance standards associated with the more mature silicon technologies do not necessarily need to be met.
Early circuit boards or base substrates used in the printing of electrical circuits, utilized insulating layers made of rigid fiberglass-reinforced resin or ceramic material. However, many printed circuit boards in use today employ flexible substrates, typically made of a polyester or polyimide material. Not only can conductive traces (wiring) be printed using known methods, but passive components (resistors, capacitors, inducers) and active components (transistors such as OFETs, switches, amplifiers, filters, electric batteries, memory, logic devices) are also known in the art. As a result, intricate electrical circuits that are printed on flexible substrates can comprise printed flexible electronics.
The aforementioned printed flexible electronics can be used in body-worn leadware. For example, as part of an electrocardiogram (ECG) procedure, a medical professional may attach electrodes to a patient and connect those electrodes to recording devices using wire leads. Leads often comprise individual wires that require considerable management to avoid becoming entangled in other devices or caught in clothing, bedding, or other equipment. Lead wires can sometimes be arranged or organized into a “harness” which keeps the wires together and more manageable. However, this lead wire harness can further complicate instances where individual leads need to be broken out to make particular connections to individual sensing points. Alternatively, the interconnection between electrodes on a patient and the recording device can be made using a device in which a conductive path is constructed (often printed) on a thin, flat substrate which can lie flatter on the patient. Such devices fall into the category known as “leadware” and provide effectively a “wearable” set of ECG lead wires to the patient. These body-worn leads can be affixed to the appropriate locations and electrodes on the patient, where they remain affixed to the patient and configured to sense ECG data. This saves the medical professional from having to constantly manage lead wires. It is also possible that the electrodes can be constructed to be integral with the leadware. Body-worn leadware is useful in myriad other applications, for example, wherever it may be desirable to measure patient vital signs or other physical characteristics, including respiration monitoring, x-rays, C.A.T. scans, fluoroscopy, and so on. Other applications for leadware can include devices for electro-stimulation for various types of continuous or intermittent diagnoses or therapies.
However, because body-worn leadware is in contact with the patient's skin, it can be uncomfortable for the wearer or can cause irritation, chafing, or scratching of the wearer's skin. The substrates, conductive layers, and dielectric layers that comprise traditional leads contain components with various organic or inorganic materials that are designed for a conductive, insulating, or semi-conductive purpose and are not necessarily designed to be comfortable to the user. Indeed they have a stiffness and sharpness of edge which, just as with paper, can lead to cuts and abrasions. Therefore, body-worn diagnostic or therapeutic devices typically require some sort of comfort layer to protect the skin of the wearer from the leadware in order to be comfortable.
Some body-worn leads have incorporated protective layers onto the printed electronically functional layers. Typically, this is achieved by the lamination of a “comfort layer” made of a woven or non-woven, spun fabric. However, traditional comfort layers suffer from problems in manufacturing, distribution, and use.
For example, the lamination of a traditional comfort layer typically requires that the comfort layer material first be coated with an adhesive and then laminated to the substrate of the flexible product. This lamination often must be carried out in register so that features of the flexible electronics themselves can be aligned with the corresponding and appropriate cut-outs in the comfort layer material. Thus, the lamination process often requires at least a secondary processing step on at least a secondary piece of equipment. Also, the secondary piece of equipment must have enough sophistication to support the necessary registration of the two layers.
Further, there is often considerable waste of the comfort layer material because the traditional comfort layer manufacturing process typically starts with a whole web or sheet (100% coverage) of material which is die cut before lamination, and then die cut along with the final printed circuit when the assembly is singulated. This is done because the various intricately-aligned features of the printed electronics can be difficult to align with the corresponding features in a pre-cut piece of comfort layer material. The fraction of comfort layer material used in the final printed circuit product is far less than 100%. Therefore, not only are additional steps incurred, thus increasing production cost, but comfort layer material is applied, then cut away and discarded during production, thus further increasing material costs.
Additionally, the packaging of traditional printed flexible electronics incorporating a comfort layer is problematic. For use with adult patients, leadware can exceed a length of 36″ long. In order to contain the leadware in packaging for distribution, typically, the product is folded over a stiffener or rolled on a spool. The rolled or folded unit and holder card or spool are then placed in a container for shipping. The conventional structure of leadware that includes a comfort layer with adhesive is such that a memory is formed, within the structure, of folds, creases, or rolls as the adhesive layer accommodates the stresses associated with any bending. As a result, bending during the packaging impresses a “kink” which the part retains throughout its useful life. For example, referring to FIG. 1A of the prior art, a flexible printed electronic part 10 of the prior art having a traditional comfort layer 12 is depicted after being unpackaged. Memory folds 14 are present in the part 10 where it was folded over its packaging insert card, even after unpacking. These memory folds can be problematic during use of the leadware. It can make the leads difficult to affix (and remain affixed) to the patient. Memory folds can also lead to more interference between the leadware and items in the vicinity such as clothing, bedding, etc. Likewise, there may be interference with the measurement of the patient's clinical information.
Therefore, there is a need for a comfort layer that can be prepared on flexible substrates and specifically, flexible printed electronics, that is easily, economically, and efficiently produced, that further does not suffer from memory fold retention problems of the prior art and can thus be easily packaged and used. Embodiments having such a comfort layer will allow more variety in packaging options, thereby adding value to its use.